1. Field of the Invention
The present invention relates to a switchback mechanism that performs switchback of a sheet and an image forming apparatus including the switchback mechanism.
2. Description of the Related Art
A typical image forming apparatus capable of duplex printing includes a discharging unit that discharges a sheet and a switchback unit that performs switchback of a sheet for duplex printing. The discharging unit and the switchback unit can be realized with two conveying rollers or three conveying rollers. When the discharging unit and the switchback unit are realized with two conveying rollers, switchback of a sheet for duplex printing is performed after the sheet has been discharged.
On the other hand, when the discharging unit and the switchback unit are realized with three conveying rollers discharging of a sheet and switchback of the sheet can be performed simultaneously. Specifically, when the discharging unit and the switchback unit is realized with first to third discharge rollers, discharging of a sheet is performed by the first and second discharge rollers and switchback of the sheet is performed by the second and third discharge rollers. Therefore, the work efficiency can be improved as compared with those composed of two conveying rollers. Incidentally, the first to third discharge rollers are arranged in such a manner that the second discharge roller is located between the first and third discharge rollers and the first and the third discharge rollers make a contact with the second discharge roller. When an image is to be formed on only one side of a sheet, after the image is formed on the side of the sheet, the sheet is discharged onto a sheet receiving tray by passing through the first and second discharge rollers. When an image is to be formed on both sides of a sheet, after the image is formed on one side of the sheet, the sheet is conveyed toward the sheet receiving tray by passing through the second and third discharge rollers. After that, the sheet is switchback-conveyed by a reverse rotation of the second discharge roller, and conveyed to a duplex-printing path by passing through the second and third discharge rollers. After the image is formed on the other side of the sheet, the sheet is discharged onto the sheet receiving tray by passing through the first and second discharge rollers.
To exercise such a duplex-printing function, a conventional image forming apparatus must be provided with a drive source capable of driving discharge rollers to rotate in any of a forward direction and a reverse direction and a branch guide for switching a conveying path of a sheet to a switchback conveying path when an image is to be formed on both sides of the sheet.
There has been developed an image forming apparatus that can simultaneously perform switching of a conveying path and switching of a rotating direction of a gear by the use of a drive force of the gear that is driven to rotate in one direction only, i.e., not in both the forward direction and the reverse direction but in either one direction (see, for example, Japanese Patent Application Laid-open No. 2007-76881). Specifically, such an image forming apparatus includes a plurality of conveying paths, three conveying rollers, a switching guide, a drive unit, and a plurality of gears. The conveying rollers respectively rotate by having contact with the adjacent conveying roller. The switching guide guides a sheet to any one of the conveying paths when an image is to be formed on both sides of the sheet. The drive unit drives the conveying rollers to rotate. The gears transmit a drive force from the drive unit to the conveying rollers. With such a configuration, there is no need to provide a drive source capable of driving the conveying rollers to rotate in both the forward direction and the reverse direction. Therefore, the configuration of the image forming apparatus can be simplified as compared with that of the conventional image forming apparatus.
However, there is still room for improvement in the configuration of the image forming apparatus.